A molecular systematic approach to explore diversity within the sellaphora pupula species complex (bacillariophyta)

The common and cosmopolitan freshwater benthic diatom Sellaphora pupula (Kutz.) Mereschk. is a model system for studying the nature of species in microalgae; the biological significance of morphological variation in this species complex has been widely demonstrated. The aim of this study was to establish a two-gene phylogeny (18S rDNA and rbcL) for 23 Sellaphora taxa, including 19 S. pupula aggregate (agg.) demes or species, S. bacillum (Ehrenb.) D. G. Mann, and S. laevissima (Kutz.) D. G. Mann. A range of analyses on separate and combined data sets indicated that Sellaphora is a monophyletic group containing four major clades. Of the traditionally recognized species, S. bacillum and S. laevissima are natural groups, but S. pupula is paraphyletic or polyphyletic because S. bacillum groups with S. pupula small lanceolate and S. lanceolata. Thickened bars at the poles of valves within the core pupula-bacillum group may be a morphological synapomorphy; the fossil record suggests that this group is at least 12 million years old. Otherwise, there was no clear pattern in the distribution of different morphologies among the major clades; each clade was also heterogeneous with respect to mating system. More intensive and extensive sampling will doubtless uncover even greater diversity; the challenge lies in its interpretation. Our results demonstrate the limitations of paleoecological, ecological, and biogeographical research based on morphospecies.